Labelled beads have been used for many years in diagnostics testing, microscope-based assays, flow cytometry-based assays and combinatorial library synthesis. They also have potential to be used in a wide range of applications including biological monitoring, security/anti-counterfeiting, optical coding, biological assays, optical data storage and sensors.
A labelled bead will generally contain a plurality of reporter moieties. The reporter moieties may, for example, be fluorescent species such as fluorescent dyes or a quantum dot. (For the purposes of the present invention a single dye molecule or quantum dot is considered to be a reporter moiety). Further possibilities include the use of magnetic particles and specifically shaped particles as reporter moieties.
Labelled beads may contain different “species” of reporter moieties. Thus, for example, a labelled bead may contain two or more different types of fluorescent dye or two or more different types of quantum dot. Depending on the number of each species present, the bead will provide a particular signal which may in effect be considered to be a “bar-code”. Put another way, there will be a first signal from the first species of reporter moiety, a second signal from the second species of reporter moiety etc. In addition the intensity of the signal from the first reporter moiety may be increased or decreased by the presence of the second reporter moiety. The particular combination of signals uniquely identifies the bead.
It will of course be appreciated that labelled beads may be produced with different combinations of reporter moieties so that different types of bead are distinguished by their “barcode” signal.
By way of illustration, such beads may be used for labelling (“tagging”) proteins or nucleic acids. Thus molecules of a first type of nucleic acid or protein may be tagged (e.g. by incubation, covalent attachment, adsorption etc.) with labelled beads providing a first barcode signal. Similarly protein or nucleic acid molecules of a second type may be tagged with labelled beads providing a second barcode signal. The tagged protein or nucleic acids may then be admixed and subjected to a particular reaction. The individual barcodes allow the product mixture to be examined (e.g. after a separation procedure) to determine the origin of the tagged molecule in other words which specific reaction sequences the bead has been exposed to or else which known molecule has been attached to the bead).
Most of the bar-coded labelled particle libraries currently available have been produced by encoding beads with different concentrations of fluorescent dyes and/or metal (including magnetic metal) particles. In the case where the moieties are fluorescent dyes, the size of the library is limited by the overlapping excitation and emission spectra of the dyes. There are also limitations on libraries produced using metal particles as the reporter moieties.
Some of the above mentioned disadvantages are overcome by the use of quantum dots as the reporter moieties.
Quantum dots are a subset of nanoparticles that when excited emit light, with quantum yields of between 30-70% and narrow emission bands across the visible spectrum and extending to both the UV and near infrared spectrum. Compared with conventional fluorescent probes (e.g. organic dyes and lanthanides) quantum dots have a number of advantages. Most notably quantum dots absorb light over a wide range of wavelengths so a number of different light-emitting quantum dots can be stimulated using a single light source to produce an output in parallel, a so-called optical barcode. This is in contrast to conventional fluorescent compounds for which (i) their absorption and emission spectra are closely coupled (thus requiring a number of different light sources for multiplexing purposes), and (ii) their emission spectrum is often extremely broad and overlapping with its absorption spectra.
Labelled beads comprised of polymeric particles incorporating quantum dots have been proposed and may be produced, for example, by including the quantum dots in a suspension polymerisation procedure. One example of such a technique is disclosed in WO 2005/021150 (The University of Manchester) which leads to beads having a size of 0.5 microns upwards.
Typically one population of quantum dot is added to a single polymerisation reaction. All polymer particles produced in a batch from a polymerisation reaction will contain statistically approximately the same number of quantum dots with the average number of quantum dots per polymer particle being determined by the reagent ratios selected in the polymerisation procedure.
Since the number of Quantum Dots per particle is relatively similar it is not trivial to distinguish between two different polymer particles produced in the same polymerisation reaction. This is, in some respects, an advantage in that the product of one polymerisation reaction can be made to be detectably different from that of another polymerisation reaction so the two batches can be used as different labelled beads. However this approach means that one polymerisation reaction produces one batch of optically coded polymer particles. By extension, to generate for example ten differently encoded beads would require ten different polymerisation reactions. For utility in optically encoded systems a large number of encoded microbeads are required which in turn would require a large number of polymerisation reactions to be carried out.
In order to overcome this disadvantage, it is possible (but by no means trivial) to separate the beads obtained in one polymerisation reaction into a number of separate fractions such that the beads in one fraction effectively provide the same signal as each other but beads in different fractions provide signals which can be distinguished from each other by appropriately sensitive equipment. Therefore, in principle, this approach means that a single polymerisation reaction can yield two or more different fractions of labelled beads that can be distinguished from each other.
It is for example possible to sort the beads (incorporating quantum dots) obtained from one polymerisation reaction on the basis of the intensity of emission of each bead using a rapid sorting procedure such as a Fluorescence Activated Cell Sorter (FACS). Here the FACS instrument would be set to include, for example, emissions between two intensity levels and to exclude emissions above and below these levels—so-called “gating”. One sorting procedure would then, for example, generate three different populations of beads. A quantity between a high and low cut-off level and two subsequent batches one above the higher cut off level and one below the lower cut-off level. However because the intensity cut-off point between one intensity level and the next is a continuum, sorting from one batch to another may result in some particles being placed in a different batch from one run to another i.e. the code fails.
This problem would be further exacerbated by the use of multiple machines where different machines will not possess exactly the same intensity calibration levels. A possible way round this problem would be to carry out a number of sorts based on intensities and simply discard particles that lie to close to a cut off point. However such an approach would be extremely time consuming (even with the phenomenal speed of FACS) and also result in the wastage of considerable amounts of materials (i.e. of encoded beads).
It is therefore an object of the present invention to obviate or mitigate the above mentioned disadvantages.